Bootstrap FileInput

In your java code just use de.agilecoders.wicket.extensions.markup.html.bootstrap.BootstrapFileUploadField instead of Wicket's org.apache.wicket.markup.html.form.upload.FileUploadField and make sure your form has form.setOutputMarkupId(true).
No additional HTML markup required. Though you could add class="btn btn-default" to make the AjaxFallback simple file input align properly in bootstrap forms.